The present invention relates to fuel cells. In particular, the present invention relates to solid oxide fuel cells. More particularly, the present invention relates to solid oxide fuel cell stacks cooled substantially by conductive/radiative mechanisms.
Fuel cells offer many advantages over conventional power generation systems. It is generally known that such devices are capable of delivering high quality electric power with greater efficiency and lower emissions when compared to comparably sized gas or diesel fed generators. Further, such systems are generally modular and can fulfill a wide range of energy needs including remote site power generation, light utility, and transportation applications as well as commercial cogeneration and residential applications.
Solid oxide fuel cells are well known devices that are capable of producing electric power at higher efficiency, however, there are a number of major hurdles including issues of operation, scale, and cost. These three factors tend to be highly correlated. Answers to materials and operational problems tend to result in difficult issues of scale and cost.
The present invention is directed generally to an electrochemical apparatus for oxidation or consumption of a fuel, and the generation of electricity, such as, a solid electrolyte fuel cell, and more particularly to fuel cells constructed of stacked plate components. Although particular embodiments are applicable to conventional co-fired solid electrolyte fuel cell apparatus, the present invention is particularly useful when utilizing non-co-fired solid oxide electrolyte fuel cells, preferably non-cofired and planar, that contain a stack of multiple assemblies or cells. Each assembly, or cell, comprises a solid electrolyte disposed between a cathode and an anode, being bounded by separators which contact the surface of the electrode opposite the electrolyte. A fuel manifold and generally two air manifolds pass gas through or over assembly elements, with gaskets sealing the anode adjacent to the air manifolds and a gasket sealing the cathode adjacent to the fuel manifold to minimize fuel and air mixing within the cell.
The fuel cell operates by the introduction of air to the cathode and the ionization of oxygen at the cathode/electrolyte surface. The oxygen ions move across the gas non-permeable electrolyte to the anode interface, where they react with the fuel flowing into the anode at the anode/electrolyte interface, releasing heat and supplying their electrons to the anode. The electrons pass through the anode and separator into the next adjacent cathode.
A key requirement in all fuel cell systems is the need for heat removal from the stack or bundle. The heat released by fuel oxidation is always significantly greater than the electric power which may be extracted from the fuel cell stack. Known fuel cell systems prefer to operate at nearly isothermal conditions in order to minimize internal thermal stresses and to achieve a good balance between stack life and performance. The typical operating temperature for a solid oxide fuel cell is about 600xc2x0 C. to 1,000xc2x0 C.
In known solid oxide fuel cell devices, the dominant cooling mechanism for heat removal from the fuel cell stack or bundle is convective heat transfer using air as the cooling fluid. In this scheme, cold air is forced through the system using a blower or compressor. The air is partially heated in a heat exchanger by hot air and/or exhaust. The preheated air (at a temperature below stack operating temperature) is then introduced to the fuel cell stack or bundle. The air flows through the stack, interacting chemically to provide oxygen to the electrochemical reaction while adsorbing heat from the internal stack surfaces. The oxygen-depleted hot air is then released from the stack and redirected through the heat exchanger outside of the stack to preheat new incoming air.
There are several issues that must be considered in this scheme. First, to minimize the thermal stress to the stack, the incoming air is preheated to a temperature not too far below the desired operating temperature. However, preheating the air limits its ability to adsorb more heat from the stack (essentially, the heat capacity and change in temperature dictate the amount of heat that can be removed). To compensate, considerably more air must be driven through the stack than is required for satisfactory electrochemical operation.
The amount of air necessary for completing the chemical reaction is called the stoichiometric ratio, or stoic. Typical planar fuel cells require 6-10 stoics of air to maintain thermal control.
The size and weight of the required heat exchanger for air preheating is proportional to this quantity of air, and further depends upon its heat duty, stream flow rates, temperature approach, and allowable pressure drops. Preheating air to the temperatures required for solid oxide fuel cells in particular requires heat exchangers made of expensive high-temperature materials. To accommodate the bulk and cost of the heat exchanger, known solid oxide fuel cell systems are generally limited to producing at least 10 kW of power. These systems are thus impracticable for smaller, lower cost applications. Additionally, these systems, because of their size and cost, are impractical for portable or mobile applications where weight and size are critical issue.
Matsumura U.S. Pat. No. 5,426,002 and Elangovan 5,480,738 describe examples of fuel cells that use conductive cooling to control the temperature in a fuel cell.
Hsu U.S. Pat. No. 5,338,622, also describes a conductive method of controlling fuel cell temperature. Hsu, however, uses a working fluid instead of the oxidant air to conduct heat from the fuel cell. While using the working fluid reduces the amount of air used, the working fluid is disadvantageous because it adds expense and requires additional components for providing it to the heat exchanger.
Thus, it is desirable to utilize a thermal control mechanism that requires less air to control fuel cell temperature. It is further desirable to reduce the size and expense of the heat exchanging equipment and thus reduce the size of the balance of systems to provide for smaller fuel cell systems, such as for residential or portable use.
We have found that an electrochemical apparatus employing relatively small sized solid oxide fuel cells using conduction and radiation heat rejection as the primary thermal control mode for the stack presents significant improvements over other known methods. These include a significant reduction in the air flow required to cool the stack, leading to more compact stacks and smaller, lighter, and less expensive balance-of-system equipment.
According to the present invention, the major mechanism of heat removal is shifted from on-cell convective cooling to conduction within the stack plus radiation from the outer surface of the stack. The thermal enclosure (surfaces surrounding the stack) is then cooled by conventional methods including heat exchange with incoming air, fuel, or auxiliary air. By shifting the stack cooling mechanism, several benefits can be realized. First, the need for excess air is dramatically reduced. The typical air flow rate is very low, being about 1 to about 3 times stoichiometric (depending on system design parameters). This greatly reduces the size, weight, and complexity of the air heat exchanger. Further, the air is typically preheated close to the stack temperature, which minimizes the thermal stresses and temperature gradients in the stack. The improved cooling method provides a number of characteristics that present unique opportunities for the solid oxide fuel cell industry.
The present invention therefore provides an electrochemical apparatus comprising: a stack of least two compact fuel cells, wherein the cell maximum thermal pathway is up to about four centimeters. The cell has a solid electrolyte layer disposed between an oxygen electrode layer and a fuel electrode layer. A separator layer contacts the surface of opposing electrodes of adjacent cells opposite the electrolyte. The cell defines internal feed passages for providing reacting gases to the electrodes and has a rim portion adapted to radiate cell heat outside the cell. At least one layer of the cell is adapted to conduct cell heat to the cell rim for transfer by radiative cooling. In one embodiment, the separator is adapted for and capable of efficient heat conduction from the interior of the cell to its rim.
The present invention further provides an electrochemical apparatus comprising: a stack of at least two compact cells; wherein the cell maximum thermal pathway is up to about four centimeters; wherein the cell has a solid electrolyte layer disposed between an oxygen electrode layer and a fuel electrode layer, and a separator layer contacting the surface of opposing electrodes of adjacent cells opposite the electrolyte. The cell defines an internal feed passage for providing one reactant gas to one of said electrodes, and the other electrode is adapted to permit a second reactant gas to flow into the other electrode. The cell has a rim portion adapted to radiate cell heat outside the cell, and at least one layer of the cell is adapted to conduct cell heat to the cell rim for transfer by radiative cooling. The cell preferably has a symmetrical planar cross-sectional shape. The second reactant gas may flow into the other electrode by gas diffusion.
The present invention further provides a method for cooling an electrochemical apparatus wherein the apparatus comprises a stack of at least two solid oxide fuel cells, wherein each cell has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, a separator contacting the electrodes of adjacent cells, wherein the cell defines internal feed passages for providing an oxygen-bearing gas and a fuel gas, the method comprising: feeding the oxygen-bearing gas to the oxygen electrode at a low flow rate; reacting the fuel gas at the fuel electrode, conducting the heat produced within the cell to a cell rim, and radiating the heat from the cell rim.